Materials for Shielding Astronauts from the Hazards of Space Radiations

One major obstacle to human space exploration is the possible limitations imposed by the adverse effects of long-term exposure to the space environment. Even before human spaceflight began, the potentially brief exposure of astronauts to the very intense random solar energetic particle (SEP) events was of great concern. A new challenge appears in deep space exploration from exposure to the low-intensity heavy-ion flux of the galactic cosmic rays (GCR) since the missions are of long duration and the accumulated exposures can be high. Because cancer induction rates increase behind low to rather large thickness of aluminum shielding according to available biological data on mammalian exposures to GCR like ions, the shield requirements for a Mars mission are prohibitively expensive in terms of mission launch costs. Preliminary studies indicate that materials with high hydrogen content and low atomic number constituents are most efficient in protecting the astronauts. This occurs for two reasons: the hydrogen is efficient in breaking up the heavy GCR ions into smaller less damaging fragments and the light constituents produce few secondary radiations (especially few biologically damaging neutrons). An overview of the materials related issues and their impact on human space exploration will be given. INTRODUCTION The ionizing radiations in space affecting human operations are of three distinct sources and consist of every known particle including energetic ions formed from stripping the electrons from all of the natural elements. The radiations are described by field functions for each particle type over some spatial domain as a function of time. The three sources of radiations are associated with different origins identified as those of galactic origin (galactic cosmic rays, GCR), particles produced by the acceleration of solar plasma by strong electromotive forces in the solar surface and acceleration across the transition shock boundary of propagating coronal mass ejecta (solar energetic particles, SEP), and particles trapped within the confines of the geomagnetic field. The GCR constitutes a low level background which is time invariant outside the solar system but is modulated over the solar cycle according to changes in the interplanetary plasma which excludes the lower energy galactic ions from the region within several AU of the sun [1]. The SEP are associated with some solar flares which produce intense burst of high energy plasma propagating into the solar system along the confines of the sectored interplanetary magnetic field [2] producing a transition region in which the SEP are accelerated. SEP have always been a primary concern for operations outside the Earth's protective magnetic field and could deliver potentially lethal exposures over the course of several hours [3]. The trapped radiations consist mainly of protons and electrons within two bands centered on the geomagnetic equator reaching maximum intensity at an altitude of 3,600 km followed by a minimum at 7,000 km and a second very broad maximum at 10,000 km [4]. The trapped radiations have limited human operations to altitudes below several hundred kilometers and potentially lethal exposures are obtained over tens of hours in the most intense regions. Low inclination orbits are shielded from extraterrestrial radiations by the geomagnetic field and are mainly exposed to the trapped environment. Inclinations above 45° are sufficiently near the geomagnetic poles for which GCR and SEP exposures can be significant. Indeed, about half of the expected exposures of the International Space Station (ISS) in its inclined orbit of 51.6° will be from GCR [5]. In the usual context, shielding implies an alteration of the radiations through interactions with intervening materials by which the intensity is decreased. This understanding is to some degree correct in the case of the relatively low energy particles of the SEP and the trapped radiations wherein the energy deposited in astronaut tissues can be easily reduced by adding shield material. As one would expect, some materials are more effective than others as the physics of the interactions differ for various materials. The high energies associated with the GCR are distinct in that the energy absorbed in astronaut tissues is at best unchanged by typical spacecraft shielding configurations and use of some materials in spacecraft construction will even increase the energy absorption by the astronaut. For GCR, one must abandon the concept of "absorbing" the radiation by use of shielding. The protection of the astronaut in this case is not directly related to energy absorption within their body tissues but rather depends on the mechanism by which each particle type transmitted through the shield results in biological injury. Even though the energy absorption by the astronaut can be little affected, the mixture of particle types is strongly affected by the choice of the intervening shield material. Knowledge of the specific biological action of the specific mixture of particles behind a given shield material and the modification of that mixture by choice of shield materials is then a critical issue in protecting the astronaut in future human exploration and has important implications on the design and operation of ISS. Understanding the biological effects of GCR behind intervening material is then key to protection in future NASA activity in either ISS or deep space. As yet no standards on protection against GCR exposures have been promulgated since insufficient information exists on biological effects of such radiations [6,7]. The most important biological effect from GCR exposure of which we are currently aware is cancer induction which relates to mutation and transformation (a specific mutation) events in astronaut tissues. Our knowledge of radiation carcinogenesis in humans is for gamma ray exposures for which excess career risk is proportional to tissue dose (energy absorbed per unit mass) accumulated at low dose rates. Although insufficient data exists to estimate astronaut cancer risks from the GCR high charge and energy (HZE) ions, there exist relatively detailed data on the biological response of several systems including survival, neoplastic transformation, and mutation in mammalian cells and Harderian gland tumor induction in mice. Other biological effects may come to light as exposure of living systems to high energy heavy ion beams continues to be studied. We will discuss the available response models in light of the design criteria used for ISS and the implications for materials research. For further discussion of these issues see "Shielding Strategies for Human Space Exploration" [8]. In the present paper, we review the GCR environment and discuss the issues of shield design in the context of developing a strategy for reducing the health risks of astronauts in future missions. In particular we will examine the role of materials research and development in controlling astronaut health risks from exposure to ionizing radiation in space. GCR AND BIOLOGICAL RESPONSES The galactic cosmic rays consist mainly of nuclei (ions) of the elements of hydrogen thru nickel. The energy spectra are broad and extend from tens to millions of MeV (figure 1). The most important energies for protection lies near maximum intensities from a few hundred to several thousand MeV/nucleon (a nucleon is the name given to neutrons and protons of which the ions are composed). The salient feature of these radiations is that a significant number of these particles have high charge which affects the means by which energy is transferred to tissues. Their ion tracks seen in nuclear emulsion are shown in figure 2. The optical density (related to energy deposited) of the track increases as the ion charge squared and the intensity and the lateral extent of the track depend on the ion velocity. The tracks in the figure are for approximately 400 MeV/nucleon ions. Considering that the mammalian cell nucleus size is